fix(files): report success cancellation and errors with safe next actions (U02)

src/Duempelkas.App/Pages/Accounts/AccountDetail.razor

@@ -7,6 +7,9 @@
 @inject NavigationManager Navigation
 
 <div class="container-fluid">
+    <OperationNotice Result="fileResult" />
+    @if (fileBusy) { <p role="status" aria-live="polite">Dateiaktion läuft … Bitte warten.</p> }
+
     @if (account == null)
     {
         <div class="text-center py-5">
@@ -35,7 +38,7 @@
             <button class="btn btn-info btn-nav" @onclick="() => showAddTransfer = true">
                 <i class="bi bi-arrow-left-right"></i> Umbuchung
             </button>
-            <button class="btn btn-dark btn-nav" @onclick="HandleExport">
+            <button class="btn btn-dark btn-nav" @onclick="HandleExport" disabled="@fileBusy">
                 <i class="bi bi-file-earmark-pdf"></i> PDF
             </button>
             |